Wash and pat dry the leg of lamb.,Cut a few slits in the upper part and put one clove of garlic into slices and push into the slits.,Pour the oil on top of the leg.,With a blunt knife, spread the mustard all over the top, making sure that it penetrates in the slits.,Salt and pepper to taste.,Place the sprigs of Rosemary and the remaining garlic in a baking pan ( you might like to add a bit of oil).,Sit the Leg of lamb onto the rosemary and bake in a preheated oven at 190c/375F (25 minutes per 500gr /1 pound).,______,Drain excess oil from pan (Remove garlic and rosemary).,Add half a glass of water (or red wine) and scrape pan with a spoon to collect and emulsify all the baked on bits.,Strain through a fine colander into a small pan, boil briskly for a couple of minutes, adding 1/3 teaspoon of corn starch, if needed.,Serve lamb with the pan juices, roasted potatoes and other vegetables of your choice